Electro-optic modulators have become one of the key components for high-speed optical transmission systems. The widely used electro-optic modulators are generally made from lithium niobate (LiNbO.sub.3) because of its high electro-optic coefficient and high-quality crystals. Still further, it is also possible to make integrated-optic modulators using lithium niobate crystals. However, lithium niobate modulators are intrinsically polarization dependent which greatly limits their applications. For example, it has been found that inline bit-synchronized phase modulation is very effective in improving system performance. However, the state of polarization in the middle of a transmission line changes over time, which makes it difficult to use lithium niobate modulators unless some automatic polarization tracking technique is used. Thus far, lithium niobate modulators are only used at a transmitter site when the laser source is still linearly polarized. The polarization dependence comes from the asymmetric electro-optic response of lithium niobate crystals. For 10 Gbit/sec and 40 Gbit/sec systems, it is desirable to use phase modulators either in the middle of the transmission line or in front of a receiver. However, lithium niobate modulators have not been able to be used due to their polarization dependency without an automatic polarization tracking technique being used, which increases the cost.
All-optical regenerators are expected to be a key element in future high-capacity photonic networks since such regenerators provide many advantages compared to their electronic counterparts. The advantages provided by the all-optical regenerators are, for example, bit rate independence, higher speeds, and lower cost. Several types of all-optical regenerators have been proposed in recent years such as, for example, semiconductor optical amplifier (SOA) based regenerators, nonlinear optical loop mirror (NOLM) based regenerators, and synchronous modulation based regenerators.
Essentially unlimited propagation distance at high bit rate (&gt;10 Gbit/sec) has been demonstrated using the technique of synchronous modulation. In this regard, see, for example, the articles by M. Nakazawa et al. in (a) Electronic Letters, Vol. 27, No. 14, pages 1289-1291, Jul. 4, 1991, entitled "10 Gbit/s Single-Pass Soliton Transmission Over 1000 km" (b) IEEE Journal of Quantum Electronics, Vol. 29, No. 7, pages 2189-2197, July, 1993, entitled "Soliton Transmission Control In Time And Frequency Domains" and (c) Electronic Letters, Vol. 29, No. 9, pages 729-730, Apr. 29, 1993, entitled "Experimental Demonstration Of Soliton Data Transmission Over Unlimited Distances". The disadvantage of the synchronous modulators used is that polarization dependence is an very detrimental limitation for practical applications, other than possibly integrated high-speed transmitters, since endless polarization tracking would be required.
The article by P. Brindel et al. entitled "20 Gbit/s Optically Regenerated Transmission over 40 Mm Based on Polarization-independent, Push-pull InP Mach-Zehnder Modulator", in ECOC '98, pages 685 and 686, September, 1998, discloses a newer type of modulator without polarization dependence. The modulator is a Mach-Zehnder modulator made from InP. However, this type of modulator has a high insertion loss (&gt;20 dB) which makes it difficult to use in practical applications.
It is desirable to provide polarization independent lithium niobate modulators for use in, for example, an all-optical regenerator which and advantageously has low insertion loss (e.g., &lt;8 dB), a low driving voltage, and can be widely used in high-speed transmissions.